Unless it orbits faster than Earth, it's periodic data too. If it does orbit the sun faster than Earth, outside Neptune's orbit, its really cruising!
Everything in the sky can be thought of in terms of how many degrees or arcseconds wide it is. The moon and sun are nearly the same 'size' in these terms, even though the sun is so much bigger in actual size.
This planet 9? It's ten times the size of the Earth (ish), but in terms of arcseconds, it's abysmally tiny. There simply aren't many telescopes powerful enough to see something that small in the sky. Worse, the smaller it is the more places in the sky you need to look to try to find it.
It's like you've dropped a single 10nm transistor on your living room carpet.